.\" SPDX-License-Identifier: MIT
.\"
.Dd December 22, 2023
.Dt RGBLINK 1
.Os
.Sh NAME
.Nm rgblink
.Nd Game Boy linker
.Sh SYNOPSIS
.Nm
.Op Fl dMtVvwx
.Op Fl l Ar linker_script
.Op Fl m Ar map_file
.Op Fl n Ar sym_file
.Op Fl O Ar overlay_file
.Op Fl o Ar out_file
.Op Fl p Ar pad_value
.Op Fl S Ar spec
.Ar
.Sh DESCRIPTION
The
.Nm
program links RGB object files, typically created by
.Xr rgbasm 1 ,
into a single Game Boy ROM file.
The object file format is documented in
.Xr rgbds 5 .
.Pp
ROM0 sections are placed in the first 16 KiB of the output ROM, and ROMX sections are placed in any 16 KiB
.Dq bank
except the first.
If your ROM will only be 32 KiB, you can use the
.Fl t
option to change this.
.Pp
Similarly, WRAM0 sections are placed in the first 4 KiB of WRAM
.Pq Dq bank 0 ,
and WRAMX sections are placed in any bank of the last 4 KiB.
If your ROM doesn't use banked WRAM, you can use the
.Fl w
option to change this.
.Pp
Also, if your ROM is designed for a monochrome Game Boy, you can make sure that you don't use any incompatible section by using the
.Fl d
option, which implies
.Fl w
but also prohibits the use of banked VRAM.
.Pp
The input
.Ar asmfile
can be a path to a file, or
.Cm \-
to read from standard input.
.Pp
Note that options can be abbreviated as long as the abbreviation is unambiguous:
.Fl \-verb
is
.Fl \-verbose ,
but
.Fl \-ver
is invalid because it could also be
.Fl \-version .
The arguments are as follows:
.Bl -tag -width Ds
.It Fl d , Fl \-dmg
Enable DMG mode.
Prohibit the use of sections that doesn't exist on a DMG, such as VRAM bank 1.
This option automatically enables
.Fl w .
.It Fl l Ar linker_script , Fl \-linkerscript Ar linker_script
Specify a linker script file that tells the linker how sections must be placed in the ROM.
The attributes assigned in the linker script must be consistent with any assigned in the code.
See
.Xr rgblink 5
for more information about the linker script format.
.It Fl M , Fl \-no-sym-in-map
If specified, the map file will not list symbols, only sections.
.It Fl m Ar map_file , Fl \-map Ar map_file
Write a map file to the given filename, listing how sections and symbols were assigned.
.It Fl n Ar sym_file , Fl \-sym Ar sym_file
Write a symbol file to the given filename, listing the address of all exported symbols.
Several external programs can use this information, for example to help debugging ROMs.
.It Fl O Ar overlay_file , Fl \-overlay Ar overlay_file
If specified, sections will be overlaid "on top" of the ROM image
.Ar overlay_file :
empty space between sections will be filled by the corresponding bytes from
.Ar overlay_file .
This is useful to patch an existing ROM.
Note that all sections must be fixed (forced bank
.Sy and
address)!
.It Fl o Ar out_file , Fl \-output Ar out_file
Write the ROM image to the given file.
.It Fl p Ar pad_value , Fl \-pad Ar pad_value
When inserting padding between sections, pad with this value.
The default is 0.
.It Fl S Ar spec , Fl \-scramble Ar spec
Enables a different
.Dq scrambling
algorithm for placing sections.
See
.Sx Scrambling algorithm
below for an explanation and a description of
.Ar spec .
.It Fl t , Fl \-tiny
Expand the ROM0 section size from 16 KiB to the full 32 KiB assigned to ROM.
ROMX sections that are fixed to a bank other than 1 become errors, other ROMX sections are treated as ROM0.
Useful for ROMs that fit in 32 KiB.
.It Fl V , Fl \-version
Print the version of the program and exit.
.It Fl v , Fl \-verbose
Verbose: enable printing more information to standard error.
.It Fl w , Fl \-wramx
Expand the WRAM0 section size from 4 KiB to the full 8 KiB assigned to WRAM.
WRAMX sections that are fixed to a bank other than 1 become errors, other WRAMX sections are treated as WRAM0.
.It Fl x , Fl \-nopad
Disables padding the end of the final file.
This option automatically enables
.Fl t .
You can use this when not not making a ROM.
When making a ROM, be careful that not using this is not a replacement for
.Xr rgbfix 1 Ap s Fl p
option!
.El
.Ss Scrambling algorithm
The default section placement algorithm tries to minimize the number of banks used;
.Dq scrambling
instead places sections into a given pool of banks, trying to minimize the number of sections sharing a given bank.
This is useful to catch broken bank assumptions, such as expecting two different sections to land in the same bank (that is not guaranteed unless both are manually assigned the same bank number).
.Pp
A scrambling spec is a comma-separated list of region specs.
A trailing comma is allowed, as well as whitespace between all specs and their components.
Each region spec has the following form:
.D1 Ar region Ns Op = Ns Ar size
.Ar region
must be one of the following (case-insensitive), while
.Ar size
must be a positive decimal integer between 1 and the corresponding maximum.
Certain regions allow omitting the size, in which case it defaults to its max value.
.Bl -column "Region name" "Max value" "Size optional"
Region name Ta Max size Ta Size optional
.Cm romx Ta 65535 Ta \&No
.Cm sram Ta 255 Ta \&No
.Cm wramx Ta 7 Ta Yes
.El
.Pp
A
.Ar size
of 0 disables scrambling for that region.
.Pp
For example,
.Ql romx=64,wramx=4
will scramble
.Ic ROMX
sections among ROM banks 1 to 64,
.Ic WRAMX
sections among RAM banks 1 to 4, and will not scramble
.Ic SRAM
sections.
.Pp
Later region specs override earlier ones; for example,
.Ql romx=42, Romx=0
disables scrambling for
.Cm romx .
.Pp
.Cm wramx
scrambling is silently ignored if
.Fl w
is passed (including if implied by
.Fl d ) ,
as
.Ic WRAMX
sections will be treated as
.Ic WRAM0 .
.Sh EXAMPLES
All you need for a basic ROM is an object file, which can be made into a ROM image like so:
.Pp
.Dl $ rgblink -o bar.gb foo.o
.Pp
The resulting
.Ar bar.gb
will not have correct checksums (unless you put them in the assembly source).
You should use
.Xr rgbfix 1
to fix these so that the program will actually run in a Game Boy:
.Pp
.Dl $ rgbfix -v bar.gb
.Pp
Here is a more complete example:
.Pp
.Dl $ rgblink -o bin/game.gb -n bin/game.sym -p 0xFF obj/title.o obj/engine.o
.Sh BUGS
Please report bugs on
.Lk https://github.com/gbdev/rgbds/issues GitHub .
.Sh SEE ALSO
.Xr rgbasm 1 ,
.Xr rgblink 5 ,
.Xr rgbfix 1 ,
.Xr rgbgfx 1 ,
.Xr gbz80 7 ,
.Xr rgbds 5 ,
.Xr rgbds 7
.Sh HISTORY
.Nm
was originally written by
.An Carsten S\(/orensen
as part of the ASMotor package, and was later repackaged in RGBDS by
.An Justin Lloyd .
It is now maintained by a number of contributors at
.Lk https://github.com/gbdev/rgbds .
